The production of organic chemicals by microorganisms is well known to those familiar with the fermentation art. Such fermentation reactions frequently produce a variety of products in dilute aqueous solutions. The expense of separating the chemicals from each other and from the large volume of water has been so great that production of chemicals by fermentation has not been able to compete with production of the same chemicals from fossil fuel sources. However, the gradual depletion of petroleum fossil fuel with the resultant increase in prices of petrochemical feedstocks has revived interest in such fermentation reactions which can convert carbohydrates that are renewable raw materials into simple organic chemicals.
Among the chemicals which can be produced by anaerobic fermentation of carbohydrates is acetic acid. However, the microorganisms which produce this acid are most productive in growth media or near a neutral pH. When attempts are made to run these fermentations at low pH where the free acid is produced, the organisms do not grow well and only a very low concentration of the acid is obtained. For this reason, alkali is added during the fermentations to maintain the pH near neutrality. As a result, the fermentations produce acetate salts rather than free acetic acid.
It is very difficult to separate the acetate salts from the dilute aqueous fermentation media in which they are produced. Moreover, the free acid is the product of interest to the chemical industry. For this reason, mineral acids are generally added at the end of the fermentation in order to convert the acetate salts to free acetic acid. This acidification produces quantities of inorganic salts as by-products which are of little value. Their disposal also increases the cost of the process.
A fermentation process has now been developed wherein the acetate salt is converted to free acetic acid by means of formic acid. The free acetic acid is obtained together with a formate salt. This salt can then be used as the carbon source in the fermentation process. Thus, the present method produces free acetic acid in good yield and high concentration without giving the useless by-products formed in the prior processes.
It has now been discovered that the lactate fermentation proceeds more rapidly and without the need for pH control if a formate salt is used with the lactate as the carbon source in the first fermentation step of that process. Furthermore, the acidification step of that process, which produces lactic acid, can be completed by the addition of formic acid. This produces a mixture of lactate and formate salts which are used as the carbon source for the first fermentation step. Thus, the present process is a valuable supplement to the lactate fermentation process.
As noted above, the fermentation of carbohydrates to acetic acid is run near a neutral pH where the microorganisms are most productive. To maintain this pH, alkali is added to neutralize hydrogen ions formed in the fermentation. The fermentation of formate on the other hand produces hydroxyl ions. If formate is used as part of the carbon source along with carbohydrates, no addition of alkali is needed to maintain the desired pH. Hydroxyl ions produced by fermentation of formate neutralize hydrogen ions produced by fermentation of carbohydrates. Thus, the present process is also a valuable supplement to the carbohydrate fermentation process.